This invention relates to methods and arrangements for catalytic conversion of oxides of nitrogen, hydrocarbons, and carbon monoxide in exhaust gases from internal combustion engines.
To reduce carbon monoxide (CO), hydrocarbons (HC) or oxides of nitrogen (NOx) in exhaust gases of internal combustion engines, precious-metal coated catalysts have chiefly been used. As a catalytically active coating, platinum is superior to other metals because platinum is catalytically active for converting CO and HC even at low temperatures of about 140.degree. C. The catalytic action of platinum is stable over a long period, and is not substantially impaired by migration of the platinum, even at high exhaust gas temperatures. The primary action of a Pt catalyst, e.g. Pt/Al.sub.2 O.sub.3, is the oxidation of CO and HC to CO.sub.2 with simultaneous NOx reduction. The catalytic reduction of NOx, depending on the catalyst used, has a comparatively high dependence on temperature, and the maximum NOx reduction rate may be at temperatures below 200.degree. C.
German Offenleggungsschrift No. 3642018 discloses a catalyst on a zeolite base for catalyzing the reaction of nitrogen oxides with hydrocarbons. The conversion of the nitrogen oxides is dependent on the hydrocarbon concentration in the exhaust gas to be treated, with only 50% reduction of the nitrogen oxides achieved at an NOx:HC ratio equal to 1. Such a zeolite catalyst may also be combined with conventional precious metal catalysts used as oxidation catalysts.
One problem with such catalysts, especially thermal catalysts, is that NOx may be partly converted into the ozone-damaging greenhouse gas N.sub.2 O.
An increase in NOx conversion, although not avoiding N.sub.2 O formation, may be achieved by a selective catalytic reduction (SCR) process using ammonia or urea as a reducing agent. The SCR process is only conditionally suitable for use with non-stationery engines, because, owing to the changing modes of operation of the engine, the danger of an NH.sub.3 release arises upon establishment of a 100% NOx reducing agent stoichiometry. In addition, during non-stationary operation, besides the catalyst, the ammonia or urea must be entrained in the exhaust gases.
At exhaust gas temperatures below 140.degree. C., the catalysts described above are essentially inactive. To provide exhaust gas purification during engine warm-up, adsorption of the pollutant components for example on a zeolite is used. The adsorbed pollutant component from the exhaust, for example, NOx, is desorbed from the zeolite surface at exhaust temperatures of about 200.degree. C. and passed to a precious metal catalyst which is active at that temperature for conversion. The problem in this case is the need to regenerate the zeolite surface, which must be ensured for long-term stability. In particular, potential zeolite surface poisoning presents a difficulty for this mode of exhaust gas purification.